In a mobile communication system, as shown in FIG. 1a, a wireless access network is typically composed of base stations (BTS) and a base station controller (BSC) or wireless networks controller (RNC) for controlling the base stations. The base station is mainly composed by units such as a baseband processing subsystem, a radio frequency (RF) subsystem, and antennas, which are in charge of performing transmission, reception and processing of wireless signals. As shown in FIG. 1b, one base station may cover different cells through a plurality of antennas.
In mobile communication systems, there are wireless network coverage problems that are more difficult to solve with conventional BTS technologies, such as, indoor coverage of high-rise buildings, coverage hole, or the coverage of shadow zone. A technique based on remote radio frequency units is a more effective solution being proposed to solve the above problems. In the distributed base station system based on remote radio frequency units, the primary radio frequency units and antennas are installed in regions that are required to provide coverage, and are connected to other primary baseband processing units in the base station through wideband transmission lines. The broadband transmission link connecting a remote radio frequency unit and a primary baseband processing unit may typically employ transmission medium such as optical fiber, coaxial cable, microwave and etc.
For the wireless signal transmission between a remote radio frequency unit (RRU) and a primary baseband processing unit (MU), it is possible to employ the analog signal transmission scheme or the digital transmission scheme. Although it is easier to adopt the analog signal transmission scheme, there will be disturbing components such as noise in analog lines, and the modulation of signals in the transmission will introduce nonlinear distortion. Therefore, the transmission distance between the remote radio frequency unit and the primary baseband processing unit is limited, usually in a range of several hundreds meters. In addition, the analog transmission is not suitable for the application of multiplexing technique, and may decrease the utilization of transmission lines. To this end, for the distributed base station system having this typical structure, CPRI (Common Public Radio Interface) provides standardization for the interface between the remote radio frequency unit and the primary baseband processing unit based on the digital transmission technique (its technical specification can be obtained from the website <http://www.cpri.info/spec.html>). At present, many commercial distributed base station system based on the standard have already been developed in the industry.
In the CPRI specification, terms “RE” (radio element) and “REC” (radio element controller) radio element controller correspond to the remote radio frequency unit and the primary baseband processing unit, and the interface between the RE and the REC is the CPRI link. In the CPRI protocol stack structure as shown in FIG. 2, the user plane is in charge of transmitting baseband I/Q data; The control and management plane mainly includes in-band signaling with higher real time requirement, and layer 3 protocol data (not defined by CPRI) carried on the layer 2 protocol HDLC (high level data link control) and Ethernet. Information including user plane data, control and management plane data, sync data and vendor specific data is multiplexed on the electrical or optical transmission line in a time division multiplexing mode.
Since the CPRI link is a kind of point-to-point link, each CPRI link will occupy one physical line, i.e., one optical fiber or one wavelength (when using the wave division multiplexing technique). Although the CPRI presently defines three line rates of 614.4 Mbit/s, 1228.8 Mbit/s (2×614.4 Mbit/s) and 2457.6 Mbit/s (2×1228.8), so that one CPRI link may transmit multiple I/Q baseband signals, since the limitation of the line rates, the number of I/Q baseband signals that can be transmitted through one CPRI link is very limited, and it is difficult to implement network connections of a large quantity of RRUs in a high-capacity distributed base station. On the other hand, although the CPRI define control and management information channels, the band width of these channels is very limited (less than 1/16 of the CPRI link rate). When it is necessary to transmit other signal links such as STM-N, ATM, 100/1000M Ethernet in the meantime, it will be difficult to transmit them through CPRI and have to employ additional transmission lines, thereby increasing the transmission cost and the wasting of optical fiber resources.